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Thread: Tachometer

  1. #1

    Tachometer

    I have a 1970 53 MY with DD 8V71Ns. My question is about instrumentation, specifically tachometers and whether or not the RPM reading can be adjusted. My port tach on the salon console clearly registrers 100-300 RPMs low compared to the one on the flybridge, dependant on engine speed. At idle, the difference is 100 RPMs. At WOT, the difference is 300 RPMs. The starboard engine tachs agree almost exactly throughout the RPM range.

    Can the tachs be adjusted or must I replace the port engine main console tach?

  2. #2

    Re: Tachometer

    There is an adjustment screw on the bottom of the tach inside. Accessible through a small hole in the bottom. You can adjust it, but I don't think you will win. Give it a try. You have nothing to loose. I didn't win. My starboard tach is correct sometimes. The longer I run the engine the more accurate it gets. I use a synchronizer and forget about it.



    BILL

  3. #3

    Wink Re: Tachometer

    Trojan, thanks I'll give it a try. Also have a Glendinning, but it is in disrepair. I don't know much about them, but have looked at the website. Suspect I have a bad synchro. Wires were disconnected when I bought the boat and I haven't got around to fixing the thing. Perhaps it's time....

  4. #4

    Re: Tachometer

    Today while running our boat, our port side tachometer stopped working. We were running at 1800RPM or so, and I noticed that the port tach went to 0 while running. I also checked the tachometer inside and it also read 0. The synchronizer still works perfectly though. I don't know anything about tachometers or how they work. I'm looking for someone who may know exactly what is wrong. Any ideas of where to start before I tear up half the boat looking for something as simple as a loose wire or something?

  5. #5

    Re: Tachometer

    Mine was the port tach... Stop using the synchro if you have the same problem. I had the port side tach wire out of the engine (charging pump, I think) pulled and the tach wire rebuilt (actually the worm gear on the motor). The tach has a square end on the engine and rounded out the plastic connection. You can buy new - but why. Just rebuild it. Took my mechanic less than 2 hours to dissassemble, rebuild, reinstall.

  6. #6

    Re: Tachometer

    Okay since we on the issue of tachometers this is my problem.

    As usual while fixing one problem I create an additional one.

    I have DD 671N and Guest Judson Syncronizers that just to let you know when the engines are in sync.

    I apparently broke one off the Starboad engine and now no tach. signal.

    Guest is out of business and no help.

    This piece attaches to the back of the engine and has a green wire attached to the outside of this piece and two other wires, one red and one yellow coming out of a wireing harness on the engine attached to the top of this device that has a red rubber cap one the top convering the yellow and red wires. (Exact piece on the other engine)

    I need this replace this whole piece.

    Other question is how can I at least make the tach. work?

  7. #7

    Re: Tachometer

    It is useful to realize that most tachs (analog or digital) are actually electrical meters with a "tachometer" dial. The signal is provided to the tach by a generator that varies output based on it's speed. If the generator is not turning due to a broken cable, obviously, there is no signal to the tach. If the generator is broken, again, there is no signal to the tach. If the electrical connections from the generator to the tach are bad/intermittent, the reading will may be erratic.

    Most "bad" electrical components are usually connection issues, not component issues.

    Digital tachs are not more accurate than analog tachs unless they are photo tachs or other methods (magnetic, etc) that are triggered by actually "counting" the number of times that the flywheel (or whatever) rotates.

    The 4 tachs on our 53MY all read slightly differently than the actual (photo-tach checked) engine RPM and they all vary as to where they are closest to correct. It might be worth checking and trying to adjust them to all read "correct" at some RPM but with a synchronizer I don't see any reason to bother.

    Re the glendinning itself. Their website is excellent and it is a very simple and reliable device and easy to fix if something fails.
    Mike P
    San Miguel de Allende, Mexico; Kent Island MD; San Antonio TX
    1980 53MY "Brigadoon"

  8. Re: Tachometer

    This complements post #7: It's also useful to remember that there are two types of tachs, mechanical and electrical. In either, there IS a mechanical connection to the engine by which rotation speed, RPM, is determined. (Maybe newer electronic control engines have an electronic interface??
    Some non Detroit engines may power a tach from the alternator.)

    In an electrical tach, the drive connection stops right at the engine mounted electrical generator;the drive "cable" really a direct connection is perhaps an inch or two long and may vary in size and shape according to model years. From there, electric wires connect to your electrical tach at helm stations.

    If the tach is mechanical, a long cable connects from the engine all the way to your helm tach meter. A wobbly tach needle typically means the mechanical cable needs some lubrication. An erratic electric tach, varying widely when engine RPM is constant, might means a loose or corroded electric wire.

    As noted above in an earlier post, when the square drive head (between the engine and either generator or tach meter) gets worn round and slips, no tach reading is a result..for either mechanical or electrical systems...
    Rob Brueckner
    former 1972 48ft YF, 'Lazy Days'
    Boating isn't a matter of life and death: it's more important than that.

  9. #9

    Re: Tachometer

    Just trying to clear things up a little. I'm pretty sure it's all electrically driven. Both port and starboard had wobbly tachometers, so maybe it was a bad connections for the port one that finally corroded off. What I don't get is why the synchronizer still works. The port side is the master. I thought that it would all be run by the same output if it's a mechanical failure. If it's electrical, then maybe a wire broke or the connector corroded off like mention before. Maybe with this information it will be a bit easier to diagnose. I will try to go to the boat after work today and hopefully the engine room has had time to cool down.

  10. #10

    Re: Tachometer

    The engine's mechanical drive cables go to the synchronizer first. This motion is then transferred to the tach generators which is then sent electrically to the gauges. The Glendinning does not care whether the tachs themselves or the tach generators are working at all because it doesn't use that for input. If the cables are connected to the Glen, it will attempt to keep the slave engine in sync with the lead. Improper adjustments can affect this...
    Mike P
    San Miguel de Allende, Mexico; Kent Island MD; San Antonio TX
    1980 53MY "Brigadoon"

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